The Ham Hopper-1:
Did you notice our last issue's front cover photo?
It was a photo of the Ham Hopper-1 Breadboard Transmitter! For more details on this
Frequency Hop Transmitter / Exciter -- read on!
In the fist two articles of this series we presented block diagrams, specifications
and general descriptions of what the Ham Hopper was to become. This article gives
complete schematic and parts list information on the breadboard Ham Hopper transmitter
(shown on our last issue's front cover). We also give the reader a complete, circuit
by circuit, description of the transmitter and a bit of theory on "how" each stage works.
The schematic below is the "heart" of the Ham Hopper. It is a schematic of the
"Julieboard," first published in 73 Magazine in August, 1993. The
"Julieboard" is a PC controlled DDS (Direct Digital Synthesizer) that
actually accomplishes the hopping in this design (see the 73 article for full details
and theory). We adopted the basic "Julieboard" design for the Ham Hopper
because it functions very well, is fairly inexpensive and was readily available.
The most recent 73 Magazine, August, 1994, carried another article by Bruce
Hodgkinson, VA3BH on a simple microprocessor controller for the " Julieboard."
It seems that this is the optimum way for the Ham Hopper to control it hopping, as
well. Thus, we are going to include an embedded microprocessor for "Julieboard"
frequency hop control, based on this latest input from Hodgkinson. More about this in
the next issue. The basic scheme we have in mind is that the Ham Hopper will still be
controlled by a PC (or Mac) -- but realtime frequency hop control will be unloaded f
rom the PC (or Mac) so that higher level protocols and other TNC/Radio setup/ control
functions can be accomplished in the host PC (or Mac.)
The "Julieboard" makes use of the Harris HSP45102PC-40 DDS IC. Harris
now has 50 MHz versions of this chip available, which will be used in the final
Ham Hopper-1 design. The Harris chip, a fast DAC and a handful of standard TTL
IC's are all that's required for our frequency hopping DDS subsystem.
Simple isn't it?
When we add the dedicated microcontroller for frequency hopping control, this
circuitry will change somewhat -- hopefully, it will actually get simpler.
IF Signal Generation and Modulation
The schematic below shows the RF (or IF) circuitry needed to mix the
output of the "Julieboard" up to the final IF output frequency.
This involves a PLL that synthesizes a fixed 40 MHz LO frequency. The VCO
of this PLL is also modulated (outside the PLL's loop bandwidth) by the
Gaussian Filtered digital data signal. Thus the radio's actual modulation
will be a approximation of GMSK (Gaussian Minimum Shift Keying).
The output from the "Julieboard" is passed through a high pass
then a low pass filter to give the effect of a wideband bandpass filter to
take out unwanted digital noise generated by the "Julieboard" and
its controller. This bandpass filtered IF output is then mixed with the
40 MHz LO signal from the modulated PLL and is then passed along to a filter
/ amplifier chain shown on the next page.
Filtering and Amplification
The Ham Hopper TX-2 schematic (on page 12) starts off with a 3 section
bandpass filter covering 51 to 54 MHz. This filter is needed to eliminate
the mixer's image frequency and other spurious products generated by
mixing the DDS output to the final IF frequency output.
The 52.5 MHz center frequency bandpass filter is followed y two stages
of Mini-Circuits MMIC amplification. The first stage of this gain is a MAR-6
and the second stage is a MAV-11. The MAV-11 puts out nearly 50 mW at this
frequency and directly drives the "final" power amplifier transistor,
a 2N5109. Bandpass tuned circuits match the input and output impedances of the
2N5109. A two stage bandpass, capacitively coupled filter follows the final
power amplifier. The last bit of filtering in this transmitter is a 3 section
pi-type low pass filter, used to eliminate any harmonics generated by the class
"AB" 2N5109 final stage.
While at first blush, it appears that a lot of circuitry is involved in
this transmitter -- it is really fairly simple and straightforward in its design.
In the final version of the Ham Hopper we will try to use a higher level of
functional integration and figure out a way to use pre-tuned or untuned filters.
These steps should make the transmitter design very robust and easy to build.
An ORCAD generated parts list for the TX-1 and TX-2 schematics follows.
Return to Table of Contents for this issue
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